This invention relates to communication microwave devices having high frequency electromagnetic energy waveguides connected to low noise amplifiers and more particularly to an improved low noise block feedhorn system including a low loss electromagnetic energy matching probe for coupling electromagnetic energy received by a waveguide type feedhorn to a low noise amplifier for signal processing.
In the past, low noise block feedhorn systems have included feedhorns to receive microwave energy. A section of waveguide has been used as the feedhorn. The waveguide is a metal pipe having a predetermined cross section usually rectangular or circular specifically designed to guide or conduct high frequency electromagnetic waves through its interior. The waveguide has a metal rod that projects into, but is insulated from the waveguide for coupling the received waves to an external circuit for signal processing. This metal rod is known as a "probe".
Thick probes, capped probes, and dielectric loaded probes or combinations of these probes have been used for matching the waveguide to the probe. Matching is connecting two circuits or parts together in such a way that their impedances (opposition to current flow) are equal or are equalized by a coupling device, so as to give maximum transfer of energy. To obtain maximum power transfer from the waveguide to the probe, a significant portion of electromagnetic energy must be in the vicinity of the probe. The thick probes, capped probes, and dielectric loaded probes are effective means for increasing the electromagnetic waves (RF power) surrounding the probe. A problem attends their use; the tangential components of the magnetic field around the probes induce energy (RF currents) on the probe, which cause conducting losses in the probe.
Further, in the use of thick probes and capped probes, the RF currents also increase with increase in the conducting probe's surface areas. The larger the conducting probe surface areas, the higher the conducting probe losses. To decrease the surface area, the dielectric coated probe was developed. However, the dielectric coating increases the concentration of the RF energy field surrounding the probe; the result is also higher conduction losses in the probe.
Higher conducting losses on the probe represent higher noise temperature in a satellite communication system. The noise temperature is the temperature at which the thermal noise power of a passive system per unit bandwidth is equal to the noise at the actual terminals. Noise is an undesired electric disturbance that tends to interfere with normal reception or processing of a desired signal. For example, it produces the small black or white spots over the entire image area of a television set.
Further, in known communication devices, after transitioning (passing microwave energy from one medium to another, e.g. air or vacuum into the inner metal conductor of a coaxial cable) from the waveguide's probe to a coaxial transmission line, a second transition between the coaxial line and a microstrip amplifier circuit (active element) has also been required. The input matching network between the probe and the microstrip amplifier circuit (the first field effect transistor (FET)) is the dominant passive loss contributor to the amplifier circuit.